1. Field of the Invention
The present invention relates to a light emitting device, and more particularly, to a light-emitting device for an AC power operation, which has an array of light emitting cells connected in series.
2. Discussion of the Background
A light emitting diode (LED) is an electroluminescence device having a structure in which an N-type semiconductor and a P-type semiconductor are joined together, and emits light through recombination of electrons and holes. Such an LED has been widely used for a display and a backlight. Further, since the LED has less electric power consumption and a longer service life as compared with conventional light bulbs or fluorescent lamps, its application area has expanded to the use thereof for general illumination while substituting the conventional incandescent bulbs and fluorescent lamps.
The LED repeats on/off in accordance with the direction of a current under an AC power source. Thus, if the LED is used while being connected directly to the AC power source, there is a problem in that it does not continuously emit light and is easily damaged by reverse currents.
To solve such a problem of the LED, an LED that can be used while being connected directly to a high voltage AC power source is proposed in International Publication No. WO 2004/023568A1 entitled “LIGHT-EMITTING DEVICE HAVING LIGHT-EMITTING ELEMENTS” by SAKAI et al.
According to the disclosure of WO 2004/023568A1, LEDs are two-dimensionally connected in series on an insulative substrate such as a sapphire substrate to form LED arrays. Two LED arrays are connected in reverse parallel on the sapphire substrate. As a result, there is provided a single chip light emitting device that can be driven by means of an AC power supply.
FIGS. 1 and 2 are schematic circuit diagrams illustrating a conventional light emitting device having light emitting cells connected in series: FIG. 3 is a schematic graph illustrating a driving voltage and a current with time in the conventional light emitting device; and FIG. 4 is a schematic graph illustrating a driving voltage and a light emission amount of the conventional light emitting device.
Referring to FIGS. 1 and 2, light emitting cells C1 to Cn are connected in series to constitute an array. At least two arrays are provided within a single chip 15 in FIG. 2, and these arrays are connected in reverse parallel to each other. Meanwhile, an AC voltage power source 10 in FIG. 2 is connected to both ends of the arrays. As shown in FIG. 2, an external resistor R1 is connected between the AC power source 10 and the LED 15.
The light emitting cells C1 to Cn of the array are operated for a ½ cycle of the AC voltage power source and the other array connected in reverse parallel to the array is operated for the other ½ cycle thereof. Accordingly, the arrays are alternately operated by means of the AC voltage power source.
However, the light emitting cells connected in series are simultaneously turned on or off by mean of an AC voltage. Thus, when the AC voltage has a value larger than the sum of threshold voltages of the light emitting cells, a current begins to flow through the light emitting cells. That is, the light emitting cells simultaneously begin to be turned on when the AC voltage exceeds the sum of the threshold voltages, and they are simultaneously turned off in a case where the AC voltage is less than the sum of the threshold voltages.
Referring to FIG. 3, before an AC voltage exceeds a predetermined value, the light emitting cells are not turned on and a current does not flow therethrough. Meanwhile, when a certain period of time lapses and the AC voltage exceeds the predetermined value, a current begins to flow through the array of the light emitting cells. Meanwhile, when the time is at T/4 while the AC voltage is more increased, the current has a maximum value and is then decreased. On the other hand, if the AC voltage is less than the predetermined value, the light emitting cells are turned off and a current does not flow therethrough. Thus, a time dining which a current flows through the light emitting cells is relatively shorter than T/2.
Referring to FIGS. 4(a) and (b), the light emitting cells emit light when a predetermined current flows through them. Thus, an effective time during which the light emitting cells are driven to emit light becomes shorter than a time during which a current flows through the light emitting cells.
As the effective time during which light is emitted becomes short, light output is decreased. And thus, a high peak value of a driving voltage may be needed to increase the effective time. However, in this case, power consumption in the external resistor R1 is increased, and a current is also increased according to the increase in the driving voltage. The increase in a current leads to increase in the junction temperature of the light emitting cells, and the increase in the junction temperature reduces the light emitting efficiency of the light emitting cells.
Further, since the light emitting cells are operated only when the voltage of the AC power source exceeds the sum of the threshold voltages of the light emitting cells within the array, the light emitting cells are operated in a rate slower than a phase change rate of the AC power source. Accordingly, uniform light is not continuously emitted on a substrate and a flicker effect occurs. Such a flicker effect remarkably appears when an object moving at a certain distance from a light source is viewed. Even though the effect is not observed with the naked eye, it may cause eye fatigue if the light emitting cells are used for illumination for a long period of time.